EphA2 receptor is a member of the recently cloned Eph family receptor tyrosine kinases (RTKs) that overexpressed in a variety of malignant tumors. The level of EphA2 receptor expression consistently correlates with the degree of malignancy. Overexpression of EphA2 in non-transformed breast epithelial line has led to tumor formation in nude mice. However, the mechanism by which EphA2 overexpression in tumor malignancy remains unclear. Our preliminary data show that overexpression of EphA2 in non-transformed breast epithelial cells MCF10A disrupts their ability to form acini-like spheroids in a three-dimentional culture. In addition, E-cadherin expression is mis-localized in MCF10A cells overexpressing EphA2 receptor. These data suggest high levels of EphA2 may promote malignant behavior through destabilization of cell-cell adhesion. Conversely, blocking EphA2 receptor activation in malignant tumor cells inhibits RhoA GTPase activation, cell migration, and tumor growth in vivo, indicating that EphA2 receptor signaling is required for tumor malignancy. Based on these data, we hypothesize that EphA2 overexpression (1) disrupts cell-cell adhesion through modifying junctional complexes, (2) promotes cell motility by modulation of Vav3-mediated RhoA GTPase activity, and (3) enhances tumor invasion and metastasis in vivo. To test these hypotheses, we specifically propose to:(1) identify mechanisms by which EphA2 overexpression disrupts cell-cell adhesion; (2) investigate whether increased cell motility in EphA2 overexpressing cells is mediated by Vav3- dependent activation of RhoA GTPase; and (3) determine if EphA2 overexpression is necessary and sufficient to promote tumor formation and metastasis in multi-stage transgenic tumor models. The success of this project will not only provide molecular basis of EphA2-induced cell invasion and metastasis, but also provide in vivo pre-clinical data on the effect of EphA2-deficiency or EphA2 overexpression on tumor progression and metastasis.